Any point on Earth can be located precisely with latitude and longitude, but calculating distance and area in that system is difficult. The United States Department of Commerce has created a set of state plane map projections, which allow land surveyors to calculate in feet or meters without sacrificing precision.
The United States is divided into more than 120 areas, each of which has a map projection tailored to its shape and average latitude and longitude. Large states have been subdivided into multiple sections. The National Geodetic Survey (NGS) maintains a website that allows one-step conversion between state plane and latitude-longitude coordinates.
Determine which state plane zone contains your location of interest. You cannot convert to latitude-longitude without this information. Enter the NGS SPC Zones website, Reference 1, select "By County" and click submit.
Choose the state and then county from the lists on the site, and click the "Submit" button. The form returns a line of information containing a four-digit zone number. For example, Boulder County, Colorado, is in Zone 0501. Record this number.
If you do not know whether to use the 1927 or 1983 version of the North American Datum (NAD), take the site default, NAD83.
Enter the NGS State Plane Coordinate website, Reference 2. Select the interactive conversion routine "SPC -> Latitude/Longitude" by clicking the link.
Select NAD27 or NAD83, whichever you previously used to find the zone number, by clicking on the proper radio button.
Input the state plane coordinates as shown in the examples. You must include a decimal place. For example, enter Enter the Y coordinate, 379890.0 in the field marked "NORTHING," and the X coordinate, 934307.0 in the field marked "EASTING." Enter the zone number, 0501, in the field marked "ZONE" and press "Submit."
Read the latitude-longitude coordinates from the results page. For the numbers in the example, the latitude-longitude coordinates are 40° 0' 33.83" N and 105° 16' 0.70" W (Folsom Stadium at the University of Colorado, Boulder).
- State plane coordinates are not the same thing as UTM coordinates. There are many sites that will convert UTM coordinates to latitude-longitude coordinates.
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